The present invention generally relates to a ventilation system for use in intensified breathing. More specifically, the present invention relates to an anesthesia ventilation system used during surgical operations where anesthesia is being delivered to a patient through an intravenous line where the ventilation system includes both an automatic ventilation system and a manual ventilation system.
Presently, anesthesia machines are optimized for delivering anesthesia to a patient using volatile anesthetic agent liquids. In such systems, the anesthetic agent is vaporized and mixed into the breathing gas stream in a controlled manner to provide a gas mixture for anesthetizing the patient for a surgical operation. The most common volatile anesthetic agents are halogenated hydrocarbon chains, such as halothane, enflurane, isoflurane, sevoflurane, and desflurane. Additionally, nitrous oxide (N2O) can be counted in this group of volatile anesthetic agents, although the high vapor pressure of nitrous oxide causes nitrous oxide to vaporize spontaneously in the high pressure gas cylinder, where it can be directly mixed with oxygen. The anesthetizing strength of nitrous oxide is seldom enough to anesthetize a patient and therefore is typically mixed with another volatile agent.
Since volatile anesthetic agents are expensive and environmentally damaging to the atmospheric ozone layer, anesthesia machines have been developed to minimize the use of these gases. To keep patients anesthetized, a defined brain partial pressure for the anesthetic agent is required. This partial pressure is maintained by keeping the anesthetic agent partial pressure in the lungs adequate. During steady state, the lung and body partial pressures are equal, and no exchange of anesthetic agent occurs between the blood and the lungs. However, to provide oxygen and eliminate carbon dioxide, continuous lung ventilation is required.
Anesthesia machines designed to deliver volatile anesthetic agents are designed to provide oxygen to the patient and eliminate carbon dioxide, while preserving the anesthetic gases. These goals are typically met using a re-breathing circuit, where exhaled gas is reintroduced into the inhalation limb leading to the patient. In such a re-breathing circuit, carbon dioxide is absorbed from the expired gases by soda-lime in a carbon dioxide absorber. Before inhalation by the patient, the inhalation gas is supplied with additional oxygen and vaporized in aesthetic agents based upon the patient demand. In this arrangement, the additional gas flow added to the re-breathing circuit can be less than 0.5 L/min although the patient ventilation may be 5-10 L/min. The over-pressure inspiration is typically carried out using a ventilator, which is gas driven and utilizes a bag-in-bottle construction. In these ventilators, patient gas is pressurized through separate, flexible rubber membranes from the ventilator gas drive, thereby keeping the ventilator clean and prevent contamination of the re-circulating gases.
Intravenously administered drugs provide an alternative to the volatile anesthetic agents. When intravenous anesthesia is utilized, the primary functionality of the anesthesia machine is no longer needed, since the vaporized anesthetic agent is no longer circulating with the breathing gases. When intravenously administered anesthetic drugs are utilized, the anesthesia machine may use an open breathing circuit where a mixture of fresh oxygen and nitrogen is provided at the rate required by the patient and the expired gases can be removed from circulation. In such an open system, carbon dioxide absorption is no longer needed since re-circulation has been eliminated. Further, the isolation between the patient gases and the drive gases are no longer needed when the ventilation gases are provided directly to the patient. Thus, an anesthesia ventilator optimized for intravenous anesthesia lacks a bag-in-bottle unit and a carbon dioxide absorber. Further, a vaporizer for the volatile anesthetic agents is also no longer needed. These simplifications reduce equipment size, eliminate much of the cleaning requirements by reducing the number of contaminated components, and streamline the manufacturing process.
The applicants' published PCT application WO 2004/067055 describes an open circuit ventilator that includes a manual ventilation feature. The open circuit ventilator is designed to be independent of any electrical power supply. For this reason, a dedicated pressure sensitive manual bag fill valve is included in the ventilation circuit to control the manual bag filling. The manual bag fill valve includes large surface area membranes and an access opening for connecting the sides of the membrane for differential pressure sensing. As stated, the manual bag fill valve controls the bag filling independent of the electrical power supply. However, since most modern open-circuit ventilators are already highly dependent upon the availability of an electrical power supply and include battery backup, the electrical power-free operation is not required in most cases.
Therefore, a need currently exists for a ventilator optimized to be used with intravenous anesthesia that utilizes an open breathing system similar to current ventilators used in intensive care. Further, a need exists for an anesthesia ventilator that includes the ability to manually ventilate the patient, such as during induction and wakeup. The present invention eliminates the need for a manual bag filling valve while providing the manual ventilation functionality to an open-circulation ventilator for optimizing the ventilator for intravenously infused anesthesia applications.